A system-on-a-chip (SOC) typically includes, among other components, several memory systems. Each memory system includes a large number of memories. As new SOC designs call for more and more memory systems and other components, statistical yield becomes a more and more significant factor in the cost of producing an SOC. Since the memory systems including several memories constitute a significant portion of the SOC, improving the statistical yield of the memories should correspondingly improve the statistical yield of the SOC as a whole.
In a conventional method for improving the statistical yield of a SOC, a repairing scheme is used. FIG. 1 is a block diagram of a system 100 in accordance with the prior art. System 100 includes a memory 102 having a redundant column, a test wrapper 104 and a processor 106. Processor 106 such as a built-in self-test and repair (BISTR) processor sends a command to test wrapper 104 to perform a test on memory 102. Test wrapper 104 stores fault information obtained from this testing. Thereafter, processor 106 analyzes the fault information. Based on the analysis a fault is determined as a repairable fault or a fatal fault. If the failures in memory 102 are on more than one column then the fault is determined as fatal. If the failures in memory 102 are on one column then the fault is repaired by substituting the redundant column for the faulty column. The substitution may be performed by substituting the address of the redundant column for the address of the faulty column in a rewritable address table.
However, the number of redundant column available is one per memory and hence, a memory with failures on more than one column is not repairable. This reduces the statistical yield of the memories and in turn reduces the statistical yield of the memory systems and the SOC including the memory systems. Process-related parameter variations such as implant gradient and device threshold variation cause margin failures which tend to be on multiple bits. These margin failures in the memories reduce the statistical yield. Therefore, there is a need for a scheme to improve statistical yield of the SOC.